High-pressure mercury vapor electric discharge lamp



March 20, 1951 e. G. ISAACS ET AL 2,545,384

HIGH-PRESSURE MERCURY VAPOR ELECTRIC DISCHARGE LAMP Filed Nov. 25, 1947 3 Sheets-Sheet 1 lnverflora George Gtgnn Isaacs, Evan Hevbert' Nebson,

' Their Arbirofneg.

March 20, 1951 e. G. ISAACS ET AL 2,545,884

HIGH-PRESSURE MERCURY VAPOR ELECTRIC DISCHARGE LAMP 3 Sheets-Sheet 2 Filed Nov. 25, 1947 200 400 600 600 Mrw.

power rd max.

7 a' 9 e! ecfrode 5 007042 0 in ems.

ITWVQIWITOTSI George Gtgnn Isaacs,

Evan Herbert' NebsoN,

by M0 W Their Alr'kofney.

March 20, 1951 e. G. ISAACS ET AL 2,545,834

HIGH-PRESSURE MERCURY VAPOR ELECTRIC DISCHARGE LAMP Filed Nov. 25, 1947 3 Sheets-Sheet a 40 u i: 36 "o 3 32 3% 28 edec frode d/mens/bn 5 H7 mms.

lnverfiors: George Gtgmn lsaacs, Evan Her'bevlr NeLson,

Their AhTOTTWG d.

Patented Mar. 20, 1951 HIGH-PRESSURE MERCURY VAPOR ELECTRIC DISCHARGE LAMP George Glynn Isaacs, Kenton, and Evan Herbert Nelson, Harrow Weald, England, assignors to General Electric Company, a corporation of New York Application November 25, 1947, Serial No. 787,982

In Great Britain January 18, 1946 2 Claims. (Cl. 176-122) Our invention relates to high pressure electric discharge devices and more particularly to high pressure mercury vapor electric discharge lamps.

This application is a continuation-in-part of our copending patent application Serial No. 317,028, filed December 18, 1946, now abandoned, and which is assigned to the assignee of the present application.

As a general matter, electric discharge lamps may be roughly classified into: glow discharge lamps, low pressure positive column discharge lamps, and high pressure discharge lamps. Our

invention is concerned particularly with lamps/ coming within the last category and more specifically to high pressure electric discharge lamps of the short-gap type. Although certain aspects of our invention described hereinafter are applicable to high pressure discharge lamps generally, we are particularly concerned with high pressure lamps of the short-gap type wherein the configuration or diameter of the arc discharge is controlled primarily by the pressure of the ionizable medium employed, or in which an inside transverse dimension or diameter of the enclosing envelope is substantially greater than the length of the arc discharge path, as contrasted with high pressure discharge lamps of the capillary type wherein the inside diameter of the enclosing envelope, or its bore, is substantially smaller than that of the arc gap length.

The most eflicient form of high pressure mercury lamp known hitherto has been the watercooled type in which the discharge takes place through a narrow tube, for example of about two millimeters diameter, and with such lamps efiiciencies as high or higher than 60 lumens per watt have been obtained; the water-cooling required is, however, usually inconvenient and, moreover, gives rise to considerable obscuration of the light; the lamps also usually require operation at about 300 to 400 volts per cm. of arc length which with are lengths of several cms. necessitates the use of inconveniently high operating voltages.

It is an object of our invention to provide new and improved high pressure electric discharge lamps.

- It is another object of our invention to provide new and improvedmethods of operating electric discharge lamps.

It is a further object of our invention to provide new and improved constructions for high pressure mercury vapor discharge lamps capable of sustained operation at high power ratings.

' It'is .a still further object of our invention to provide a lamp structure of the short-gap type in which efficiencies higher than 60 lumens per watt can be obtained, except possibly in those lamps adapted to dissipate less than 500 watts. It may be stated here that high efficiency is more diflicult to attain with lamps adapted to dissipate relatively low wattages.

Generally speaking, in accordance with our in: vention we have found that in order to obtain substantial improvements in efficiency over and above that obtainable in prior art. discharge lamps, without sacrificing the desired long life, the discharge lamps of this type should be constructed in accordance with definite relationships between; the power transmitted to the lamp per unitof 'arc length established by the electrode spacing, the voltage per unit of arc length as established by the same spacing, and the arc length or electrode spacing determined in accordance with predeterminedranges of maximum and minimum values correlated to the power supplied to the lamp terminals, A decrease in volts per cm. generally leadsto a decrease in efliciency, but the effect is however small aslong as the volts per cm. are sufiiciently high. A further feature involves the utilization of electrodes of predetermilfledv size commensurate with, or a function of, the power supplied to the lamp, In accordance with ourinvention a high pres sure mercury vapor lamp adapted to operate without water cooling and to dissipate in normal operation a wattage, lying between 100 watts and 15,000 watts is constructed so that the arc length d lies within or between predetermined mini-; mum and maximum values for particular wattages as stated hereinafter, and the volts per cm.

I of arc length when thelamp is operated at a specified wattageQis not less than 20 for wattages greater than2000, and not less than 30 for wattages less than 2000. That is, the voltage" per cm. of are length should not'be less than 20.

Referring now'particularly to Fig. 1, we haveillustrated our invention asapplied to a high" pressure mercury vapor lamp l comprising a pair of spaced electrodes 2 and 3 constructed of a re-" fractory metal, such as tungsten, establishing between the terminals or are supporting surfaces" thereof an- 'arcgap of length d. The'electrode's 2 and 3 may be tapere d'at the endsthereof'in the manner illustrated, and these are supporting surfaces may be activated with airefractory material such as thoria. Other activating m-aterial, such asbarium or strontium oxides, may be provided away from the terminal surfaces, in well known manners, to assist inthestarting o'pt-t eration of the lamp. One such arrangement for activating the electrode or cathode may be a tungsten helic 2a tightly wound about the cathode spaced from the are supporting surface thereof.

For a better understanding of our invent on reference-maybe had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Fig. 1 illustrates one modification of our invention wherein a high pressure electric discharge lamp of the short-gap type is positioned in and supported by an external lamp structure comprising an outer enclosing envelope. Fig. 1a is a modification of the lamp shown in Fig. 1 intended for operation on direct current. Figs. 2-5, inclusive, are curves representing the optimum lamp characteristics to be employed in constructing lamps in accordance with our invention. Fig. 3 is representative as to the efficiencies whichlare obtainable by employing our invention in. the construction of high pressure lamps. Fig. $1.8 .a curve showing the relationship between lamp Power rating and the rangenof electrode spacings, wherein the ordinates represent the logarithms of thewattage to the base I0, and the abscissae represent the maximum and minimum values for the electrode spacing. In Fig. 5, the curve represents the relationship between the logarithm to thebaseill! of the lampiwattage and the electrode size .or diameter.

Electrodes ,2. and .3. arepositioned in .and supportedbya .quartzenvelope construction 4. :Electrodesl and 3 .are preferably .of cylindrical form havingsubstantialouter endparts'E and 6 extendi s-appreciable distances into the seal constructions. land at.each end of the envelope. If desired. the electrodes 2 and .3, or more particularly the ends ,5..and I6 .may be connected toexternally accessiblev terminals ,constituted by conductors 9 and. .lil which are electrically .connected to electrodes .2 and 3 throughstrip sealconstructions comprising.v metallic members II and I2 which are. embedded in the quartz ,andsealed thereto.

By .the term .id-ischarge lampof the short-gap txpe we-nmean an electric .discharge lamp in which. an inside dimension .of .the arc chamber defined by envelope 1! .isv larger than .the arc-gap length :d. Referring to Fig. l, whichis an exampleof oneshapeof envelope whichmay be employed, the. inside transverse .dimension or .diameter of. the envelope .4: in. a 'plane a::c intermediate the arc-supporting surfaces. of electrodes 2 and .3 is substantially larger than the arc-gap lengthld. It will .be-appa-rentthat agreat variety of envelope shapes may be employed in .carrying out our invention.

We have found that precautions should .be taken to {prevent instability of operation from arising-due'to thefconvection fiareof the are discharge atmosphere during operation. To this endthesdiameter of theenvelope 4 should .be :as small possibleconsistent with a desirable limitatipn'inft-he heatingrof the quartzemployed in the envelope. 'Where the rlamp is intended. to-be Qpemtedin a vertical position the uppermost part nf-the-envelope-d should .be as near as :possible to' :the discharge path without causing overheating-10f: the quartz. Alternatively orin addi tion :to :the above consideration, a shield (not shown) :may beprovi'ded around-the upper electrode :2'iintermediate its are supporting surface and :the uppermost part of the envelope v2 and preferablyan. appreciable distance therebelow, in order .to counteract and .control the .zabove-mentioned convection flare when the lamp is intended for operation in a vertical position. Such a shield may be of a refractory metal, and in some instances even of quartz, since devitrification of a quartz shield is not a limiting factor and inasmuch as such a shield would notibe subjected to a net gas pressure. The shield may take the iform of a dish arranged with its concave surface downward.

The lamp I may be enclosed within an outer protecting envelope I3. This envelope may be coated with a fluorescent material and furthermor-emay-be constructed of a vitreous composition having desiredlight transmitting characteristics. The outer envelope I3 may define an evacuated space 'betweenzitand'the lamp I which serves to vequalize:thertemperature distribution of the lamp I. As a means for supporting the lamp I, we may employ a suitable structure wherein the outer glass envelope I3 is provided with a stem tube I4 and exhaust tube-through which the envelope I3 may be exhausted. The supporting structure-for the lamp which comprises a conductive support It bent into a substantially U or rectangular shape frame having its upper end sealed into a stem pressl 5. One end of the wire support I5 is attached to an outer lead'wire I-I whichin turn is attached to the screw-threaded-shell '18 having a skirted base 19. The otherend of the support wire I:=-termina'tes in the stem press It. The lower end of the frame 1 5 is'braced by a substantially semi-circular and preferably resilient wire 2 al located atthe bottombf the envelope I'3. WireZil is attachedtoithelower end-of the frame I 5 by a wire 21 which is welded at its middle to conductor "IB'constituting the lower external terminal ofthelamp I. In this-manner, electrical connection to theelectrode 3 is established through support --I-5 and-lead wire I'I. Electrical connection to-electrode 2 andmechanical support forthe-upper-part of lamp 1 is obtained by connecting conductor 9 to a leadin-wire 22.

Considering now more specifically the characteristics of the lamp I,'we preferably employ an ionizabl'e medium such as-mercury, indicated at the bottom of the' envelope 2 as a globule '23. The amount of mercury employed may be in 'excess of that evaporated -or vaporized during the normal operation ofthe lamp. The term mercury does not exclude the possibility of" the presence oi-small orother substances which-may contribute to the light from the discharge.

In carrying out our invention'we have provided high pressure vapor lampsof the short-gap type which afiord efficiencies substantially greater than that of lamps developed heretofore and which are capable of sustained operation at high power ratings. "For example, improvemen'tsfin efiiciencies of 60 lumens per watt, or more, are obtained by constructing the lamps in accordance with the factors described hereinafter. In constructing lamps according to our invention we have found that it-"is necessary that the :arc length should be neither too long nor too short. If-the arc length, that is :the spacing between the are supporting surfaces' of 'the electrodes '2 and a3, is too short the electrode losses form too large a part of thetotal voltage, and moreover. a larger fraction of "the alight is obscured by the electrodes. If the arc length'tisitoo :long for a :given overall powersupplied to the lamp, :the intrinsic watts per cm. ofarci'leng'this .too low .to'provide the desired high efiiciencies. ThBifOllOWillg factorsorvfeaturesghave been fpnndto be important in providing :lamps having e'fiiciencies greater Wattage d (max.) (1 (min) A still further factor, shown in form of a curve in Fig. 5, and which may be employed in constructing lamps in accordance with our invention, is the electrode diameter a in mm. which is correlated to the power input W in watts to the lamp in the following manner:

These values of electrode diameters in mm. have been found optimum for operation on alternating current. For operation on direct current, the cathode is preferably smaller and the anode larger than the above stated values. When the electrodes are not of circular cross-section, the term diameter is to be understood to indicate an area having the same value as that corresponding to a circle of diameter a. For operation on alternating current the electrodes may be tapered, for example at an angle of about with respect to sides of the electrodes and provided with terminal surfaces of small radii. For operation upon direct current the cathode may be shaped as described and the anode may have a flat end for an are supporting surface and be provided with chamfered edges.

Observed improvements in efficiency of lamps constructed in accordance with our invention greater than or equal to that defined by a curve passing through 60 lumens per watt at 450 watts,

and 50 lumens per watt at 125 watts have been obtained. This curve tends toward zero lumens at zero watts, and remains at or above 60 lumens per watt above 450 watts. The above values refer to a photometric standard by which theefficiency of the well known 5100 watt street-lighting lamp is given as 45 lumens per watt, the arc length being 16 cm. and the voltage across the lamp 120 to 160 volts. The overall efficiency set forth is obtainable when the lamp is operated on direct current, or in an inductive circuit on alternating current of substantially sinusoidal wave form of commercial frequency.

The intrinsic efficiency of the lamps with which our invention is concerned depends in a primary manner upon the correlation of the watts per cm. and the arc length. It has been found that the efficiency depends to some extent upon the volts per cm. of arc length, a decrease in volts per cm. generally leading to a decrease in efficiency. This latter effect is considered to be secondary to the relationship involving watts per cm. of arc length and the arc length.

A lamp constructed in accordance with our invention and similar in general arrangement to that shown in Fig. 1, except that it is designed for direct current operation, and having its axis vertical is designed to dissipate 700 watts in normal operation. The envelope 4 is made of quartz having walls about 2.5 mms. thick. An anode and cathode such as that shown in Fig. 2 are employed, bothof which'are formed of tungsten rods, four mms. in diameter, set into suitable seals, such as strip seals. 7

The cathode is preferably tapered, as explained above, at an angle of about.20 for the last five mms. and is rounded off at the endto form a tip of small radius. The anode has a flat end provided with chamfered edges. Around the cathode, spaced away from its end, is arranged a tightly-fitting helix 2a of tungsten wire, coated with electron emissive material, which forms a starting electrode.

The distance d between the anode and the cathode, i. e. the arc length, is about 1.5 cms., and the dimensions p, q and r are approximately 60 mms., 3O mms., and 30 mms., respectively.

Mercury and a starting gas such as argon are used, the quantity of mercury being such that about 700 watts are dissipated, the voltage between the electrodes during operation being about 250 volts. The efficiency is about 70 lumens per watt.

In Fig. 2 we have there illustrated a modification of our invention as applied to a high pressure mercury vapor lamp 24 particularly suitable for operation on direct current. A cathode 25 and an anode 25 are enclosed within a quartz envelope 2?. These electrodes may be cylindrical shaped members of refractory metal, such as tungsten, sealed to the envelope 2? and extending through seal constructions 28 and 29 to provide external connections directly thereto, thereby reducing electrode and connection losses. An activating helix 25a, similar to 2a, may be employed.

Where a lamp is intended for operation on direct current, the area of the cross-section of the cathode 25 is preferably smaller than that stated above and shown in Fig. 5, and the area of the anode 2.6 is larger than the optimum values stated above. The lamp shown in Fig. 2 is constructed to dissipate 1750 watts. The envelope wall thickness is about 2.5 mms., but the width of the envelope, i. e. the dimension 7 is now about 45 mms. The dimensions 10 and q are 60 mms., and 30 mms. The arc length is 1.5 cms., and the mercury filling is such that the lamp dissipates 1750 Watts; the lamp operating voltage is about 225 volts, and the efficiency is about 70 lumens per Watt.

As stated above, the terminal surface of each of the electrodes for operation on alternating current may be activated with a refractory material such as thoria. One preferred embodiment of an activated electrode is that disclosed and claimed in U. S. patent application of Victor J. Francis, filed February 25, 1947, Serial No. 730,- 805, now Patent 2,460,738, and which is assigned to the assignee of this application. An alternative cathode or electrode construction may be used corresponding to that disclosed and claimed herein are claimed incur application Serial N 0.

177,252, filed August 2, 1950,, which, is a division cit-hepresent application.

What we :claim as new and desire to secure hy-Letters Patent of the United States is:

,1. In a high pressure mercury vapor electric discharge lamp .of the short gap type for operation at a power inputof 4000- watts, the combination comprising an :enclosing envelope, a :pair of electrodes {defining therebetween :an arc gap of a length lying within the range .:f;r om 1.7 :to 5.5 cm., inclusive,and operable at a voltage of'not less than 20 volts per centimeter of arc length at a luminous efficiency of 60 lumens per watt-orgreater.

2. In a high pressure mercury vapor electric discharge lamp of the short-gap type for operation at a power input of 4000 watts, the combination comprising an enclosing envelope, and a pair of tungsten electrodes defining therebetween anarc gap having a length lying within the range from 1.7 to 5.5 cm., inclusive, the diameters of said electrodes being greater than about 7 mm., and operable at a voltage of not ilessj'than 20 :volts- ,per centimeter :of arc length at a: luminous refliciency :of' 60. lumens per. .watt or greater.

GEORGE. GLYNN .ISAACS. EVAN HERBERT NELSON.

REFERENCES CITED The following references are of record in-the file of this patent:

UNITED STATES PATENTS Number Name Date 2,087,735 Pirani et al. July 20, 1937 2,135,734 Rompe et al Nov. 8, 1938 2,152,994 Hanlein et al. Apr. 4, 1939 2,215,300 Ryde Sept. 17, 1940 2,272,467 Kern et al. Feb. 10, 1942 2,275,768 Kern et al. Mar. 10, 1942 FOREIGN PATENTS Number Country Date 485,489 Great'Britain May 20, 1938 501,854 Great'Britain Mar. 7, 1939 OTHER REFERENCES Electric Discharge Lamps, Cotton-Chapman and Hall Ltcl., 37 Essex- Street W. 'C. 2, 1946, pp. 358-377, inclusive. 

